The present invention relates to a lithium secondary battery.
Recent years have witnessed the demand for batteries growing tremendously with development and spread of portable appliances such as personal computers and cellar phones. Especially in the lithium battery, researches have been actively conducted in various quarters as a power source which has a high energy density, because lithium is an element small in molcular weight yet capable of releasing a large amount of ionized energy.
Hitherto, the positive electrode active materials used for such a lithium battery were MnO.sub.2, V.sub.2 O.sub.5 and the like which could generate an electromotive force of the order of three volts. In recent years, a lithium battery of the order of four volts based on LiCoO.sub.2 has been commercialized.
In seeking to further raise the battery power, researches have been carried out in active materials for a positive electrode which generate a higher electromotive force. The following substances have now been reported as ones with an electromotive force of 4.8 volts versus metallic lithium: oxides of lithium manganese with a spinel structure such as LiNiVO.sub.4 (G. T. Fey, W. Li, and J. R. Dahn, J. Electrochem. Soc., vol. 141, 2279 (1994) of Reference 1); LiCr.sub.y Mn.sub.2-y O.sub.4 (C. Sigala, D, Guymard, A. Verbaere, Y. Piffard, and M. Tournoux, Solid State Ionics, vol. 81, 167 (1995) of Reference 2); LiNi.sub.x Mn.sub.2-x O.sub.4 (Q. Zhong, A. Banakdarpor, M. Zhang, Y. Gao, and J. R. Dahn, J. Electrochem. Soc., vol. 144, 205 (1997) of Reference 3); and LiMn.sub.2-x-y Ni.sub.x Cr.sub.y O.sub.4 (Y. Todorov, C. Wang, B. I. Banov, and M. Yoshio, Electrochemical Society Proceedings, vol. 97 of 18, 176 (1997) of Reference 4); and phosphates with an olivine structure such as LiCoPO.sub.4 (Japanese Laid-Open Patent Publication Hei 9-134724) of Reference 5).
In lithium batteries made with those positive electrode active materials of References 1 to 5, however, the electrolyte is exposed to a strong oxidation environment in the area where it comes in contact with the active materials, because those positive electrode active materials generate a high electromotive force. As a result, the lithium secondary batteries of that kind obtained present a number of problems.
In Reference 1, it is reported that since the oxidation reaction of the electrolyte takes place as secondary reaction, the discharge capacity decreases. References 2 and 3 indicate that because of a similar oxidation reaction of the electrolyte, the charge and discharge efficiency is low. Reference 4 points out that because of unstable electrolyte, the self-discharge of the battery is large.
Those observations are said of the compounds of the spinel structure. Lithium batteries made with compounds of the olivine structure, too, are small in discharged quantity of electricity as compared with charged quantity of electricity, that is, low in charge-discharge efficiency as shown in FIG. 5 of Reference 5.
As described, lithium secondary batteries made with those high voltage generating active materials for the positive electrode tended to self-discharge with the decomposition of the electrolyte and were low in charge-discharge efficiency. Furthermore, while not mentioned in the references, the decomposition of the electrolyte can deteriorate the long-term reliability resulting in shortened charge-discharge cycle life and other problems.